Programmed cell death is critical in growth, differentiation and homeostasis of multicellular organisms. The morphologic expression of programmed cell death, apoptosis, has a genetic basis that is clearly dependent on the BCL-2-family proteins. This family of proteins determines an early common decision point in programmed cell death. Structural studies on pro- and anti-apoptotic members of this family indicate they are related to diphtheria and colicin toxins. These proteins insert into membranes, form ion channels and participate in protein translocation. We and others have characterized the channel forming activities of the BCL-2-family members. Studies concerning these proteins have focused on the possibility that they play an important role in increased mitochondrial permeability and in release of cytochrome c. In studies preliminary to this proposal we have demonstrated that BAX forms large water filled pores in artificial liposomes. These pores undergo a concentration dependent dimer to tetramer transition, increasing their size sufficiently to activate cytochrome c transfer across the lipid bilayer. This transition occurs at nanomolar BAX concentrations, in physiologic salt and at physiologic pH without the addition of other proteins, tn this proposal we plan to study and characterize the molecular basis for formation and activation of the large BAX and BID pores, including kinetics, stoichiometry, and the effect of interaction with anti-and pro-apoptotic family members. We will also determine the effect on pore activation of the mitochondrial membrane environment.